Mode scramblers for optical fibers have found many important uses. For example, mode scramblers prevent deviation in transmission characteristics at various points along the fiber and also provide for the propagation of all the possible modes. Mode scramblers are becoming increasingly important in optical fiber Local Area Network applications to ensure that all terminals connected to a fiber bus can share information with each other. Mode scramblers mix the light modes carrying the communication so that each terminal receives an equal share of the various modes. In U.S. Pat. No. 4,676,594, one type of mode scrambler is achieved by creating a deformation such as a groove or notch on one side of a multimode or graded-index optical fiber orthogonal to the longitudinal axis thereof. The groove or notch can be produced by any suitable etching or machining process and extends through the cladding layer and at least very slightly into the outer surface of the optical fiber core The depth of the groove or notch into the core is controlled by monitoring the mode pattern in the multimode fiber during the etching or machining process to provide maximum mode scrambling. A problem with this type of mode scrambler is that light is lost when the depth of the groove extends into or through the cladding and, in particular, when the groove extends into the core's surface.
Another arrangement for an optical fiber mode scrambler is disclosed in, for example, the articles "Measurement of Baseband Frequency Response of Multimode Fiber By Using A New Type Of Mode Scrambler" by M. Tokuda et al in "Electronic Letters," Mar. 3rd, 1977, Vol. 13, No. 5 at pages 146-147; and "Transmission Characteristics Of Multimode W-Type Optical Fiber: Experimental Study Of The Effect Of The Intermediate Layer" by K. Mikoshiba et. al. in "Applied Optics," Vol. 17, No. 17, Sept. 17, 1978, at pages 2836-2841. In these articles the disclosed mode scrambler includes an optical fiber comprising specific sinusoidal serpentine bends.
Another form of an optical fiber mode scrambler is disclosed in, for example, the articles "Mode Scrambler For Optical Fibers" by M. Ikeda et al. in "Applied Optics," Apr. 1977, Vol. 16, No. 4, at pages 1045-1049; and "Mode Scrambling Can Enhance Fiber Optic System Performance" by S. L. Storozum in "Electronics," Feb. 24, 1981, Vol. 4, No. 54, at pages 163-166. The mode scrambler in these articles comprises a few fibers in a bundle surrounded by a heat shrinkable tube which when shrunk causes fiber mircrobending.
Star couplers that provide mode transfer and enhancement are also well known and comprise a first plurality of fibers interconnected to a second plurality of fibers by means of a mixing element. The mixing element is either a planar waveguide or waveguides twisted and fused together. For example, see U.S. Pat. Nos. 4,362,357 and 4,449,783 or Fundamentals Of Optical Fiber Communications" by M. K. Barnoski, 2nd Edition, Academic Press, 1981, at pages 337-339.
U.S. Pat. No. 4,749,248 discloses devices for coupling optical power into and/or from optical fiber. This patent discloses the use of a photorefractive effect that changes the refraction index of a single mode fiber's core and transfers energy from bound modes to so-called tunneling leakly modes (TL). The TL modes can then be removed from the cladding of the fiber guide with the aid of a dielectric body in contact with the fiber at a downstream point that has a refractive index approximately equal to or greater than the cladding index of refraction. The photorefractive effect requires that the core be treated with an appropriate dopant (e.g. Fe or Bi) and constantly exposed to light of the appropriate wavelength. If the light is removed, the index of refraction returns to its original state. In addition, this patent proposes the use of a photoelastic effect that varies the refractive index of the core by light that induces stresses into the core.
The problem remaining in the prior art is to provide a non-invasive, yet permanent mode scrambler in a multimode optical fiber which provides the desired mode scrambling with a minimum light loss.